Polymers have been utilized in various fields not only as general-purpose structure forming materials but also as high value added type materials having high function or performance. Following this, importance in manufacturing a polymer material on the basis of a precise design is increasing. Cage-type silsesquioxane derivatives having a dimethylsiloxane group are watched as an organic/inorganic composite material containing a silsesquioxane as an inorganic component. This is because they are expected to be utilized for precursors of organic/inorganic hybrid materials, low-dielectric materials, optical crystals, materials for forming a liquid crystal display element, and the like, and the reason for this resides in the matter that the silsesquioxane derivatives have a structure closed to silica or zeolite. Then, there are reported cage-type silsesquioxanes in which a hydroxyl group, an epoxy group or a methacryloyloxy group is bound to a dimethylsiloxane group (see Non-Patent Documents 1 to 3). So-called organic/inorganic composite materials between an organic polymer and a silsesquioxane are prepared by utilizing such functional groups. Organic/inorganic composite materials can be obtained by radical polymerizing a cage-type silsesquioxane having a methacryloyloxy group singly or in the co-presence of an acrylic monomer.
In order to optimize a function of a polymer material depending upon the purpose, it is necessary to precisely analyze molecular properties of a polymer or properties as a molecular agglomerate. For that reason, a polymer having a distinct structure must be used. However, conventional organic/inorganic composite materials including the foregoing composite materials do not contain a structure-controlled polymer as an organic component. Since the majority of organic/inorganic composite materials are obtained by mechanical blending of a silsesquioxane and an organic polymer or other means, it was extremely difficult to control the structure of the composite as a molecular agglomerate. Then, it was attempted to control the structure of the polymer by using a polymerization initiator. Non-Patent Document 4 discloses that an α-haloester group is a good initiator of living radical polymerization for a styrene based monomer and a methacrylic acid based monomer. However, any silsesquioxane derivative having an α-haloester group has not been known so far.    [Non-Patent Document 1]
J. Am. Chem. Soc., 122 (2000), 6979    [Non-Patent Document 2]
Chemistry of Materials, 8 (1996), 1592    [Non-Patent Document 3]
Macromolecules, 29 (1996), 2327    [Non-Patent Document 4]
Chem. Rev., 101 (2001), 2921
An object of the invention is to provide a novel silicon compound having a living radical polymerization initiating ability for addition polymerizable monomers of a wide range and a polymer obtained by using the same, thereby solving the foregoing problems regarding the conventional organic/inorganic composite materials.